http://www.howland.co.uk International Geomorphology 1986 Part I Edited by V. Gardiner © 1987 John WHey & Sons Lld THE NATURAL PROCESS OF DEGRADATION OF AN OVERCONSOLIDATED CLAY UNDER DIFFERING CLIMATIC CONDITIONS AS ILLUSTRATED BY THE WEALD CLAY AF Howland AF Howland Associates, Barrington, Cambridge, England ABSTRACT The Lower Greensand escarpment between Hythe and Aldington, Kent is an abandoned marine cliff. This section of slope illustrates the process of natural degradation under wet temperate climatic conditions. A comparison with inland slopes which have been affected by solifluction activity during former perglacial climatic conditions has identified fundamental differences in the slope processes between the two. Conditions unique to the periglacial environment are required for the low angled slope processes which develop, but once the shear surfaces are produced movement can be initiated on them, even under the present wet temperate climate, by a modification of the slopes brought about by modern civil engineering development. An understanding of the processes and conditions that have influenced the development of slopes is paramount to the efficient engineering design on or close to them. Geomorphological relationships can provide a useful and rapid tool in the consideration of such developments and can have particular advantages where large areas are involved. INTRODUCTION Much of southern and eastern England is underlain by geological materials which can be described as 'soils' with regard to their engineering behaviour. Many of these are overconsolidated clays which were laid down at various periods in the geologic past. These include the London, Gault, Weald, Kimmeridge, Oxford and Lias Clays, also those clayey units within the Barton and the Woolwich and Reading Beds as well as the more recent boulder clay covering extensive areas of East Anglia. 139 Copyright Protected http://www.howland.co.uk 140 A.F. Howland Probably the most important influence on slope formation has been climatic changes in the recent geological past which have affected both slope stability and degradation. If the toe of a slope is steadily removed by erosion and that erosion subsequently ceases, the slope form will degrade under natural processes involving landsliding and soil creep (Chandler 1970a, Hutchinson 1967, 1975, Savigar 1952, and Skempton and Hutchinson 1969) such that the overall slope angle will reduce with time to some ultimate angle of stability. This angle will be affected by a variety of factors but for the relatively uniform character of the overconsolidated clays of south east England the strength of the material and in particular its angle of internal friction will play an important part (Skempton and Delory 1957). Vegetation may also have a marked effect on stability. Crozier (1979), Hutchinson (1967) and Skempton and Delory (1957) have described how deafforestation can spark off instability in previously stable slopes. Chandler (1972) and Weeks (1969) have described slopes which are flatter than the ultimate angle of stability for the present prevailing conditions, but which contain slickensided surfaces indicating that movement has previously occured on them. Clearly for these slopes to have developed conditions must have been different during their formation to those acting today, or some other process than mass movement was involved. Consequently, the 'stability' of any slope can be regarded as no more than a temporary state which can change in response to the conditions which act on it. An understanding of the factors and conditions which prevail during the formation of such slopes is often crucial to the satisfactory civil engineering development in their vicinity. Part of the Weald Clay outcrop in Kent, England, is on the north side of the Romney Marsh. Before the formation of the marsh the outcrop, capped by the Hythe Beds, formed a line of sea cliffs between Aldington in the west and Hythe in the east (Fig 1). As the marsh silted up it protected the cliffs from basal erosion by the sea and allowed them to degrade naturally to a stable slope angle. The marsh has developed progressively from the west as a response to the growth of the Dungeness Foreland (Gallois 1965, Lewis 1932). Since the processes of natural degradation are time dependent the progressively decreasing period of protection along the cliff means that the slope illustrates many of the phases of degradation (Howland 1974). Much of the abandonment has occured since the last Ice Age, prohably in response to the general rise in sea level. Therefore this particular section of the escarpment and the processes of degradation which have acted on it have not been influenced by periglacial conditions and afford an Copyright Protected http://www.howland.co.uk Degradation of an overconsolidated clay 141 opportunity for comparison with inland slopes on which mass movement processes under periglacial conditions predominate. This paper summarises the change in surface character of the abandoned slope, compares this to otherwise similar slopes inland and discusses the implications that these have on the processes of natural degradation. THE GEOLOGY OF THE WEALDEN DISTRICT The Wealden District covers much of the counties of Kent and Sussex in south east England. It forms a low-lying fertile vale about 130 km by 40 km bounded by the escarpments of the North and South Downs. Structurally the Weald forms an area of inverted relief being an eroded anticlinorium thrown up at the edge of the Tertiary Alpine Orogeny and carved out by a marine transgression­ during the Pliocene. The landforms have been subsequently modified by subaerial weathering and erosion to forma concentric pattern of Cretaceous sandstones and clays forming escarpments and intervening vales. The Weald Clay forms the upper horizons of the Wealden Series within the Cretaceous sequence. It increases in thickness along its outcrop from 120 metres at Hythe to 450 metres at Guildford and is the most argillaceous of the Series. In a weathered state it is a light brown to grey, often mottled heavy clay or silty clay while unweathered it is a dark brown or grey overconsolidated fissured clay which can have a shaley texture. Horizons or nodules of mudstone are found within the unweathered material and the presence of sand has been noted. To the south of Maidstone the Weald Clay becomes somewhat more variable. Although it still consists mainly of clays and shales, the megacyclothems identified by Allen(1959) are more evident. This produces a large scale regular progression of slightly differing lithologies within the general sequence of the clay. Lying un conformably on the Weald Clay is the mainly bluish-grey and brown, mottled sandy clay and pale-grey, slightly glauconitic Atherfield Clay. The lower horizons at the junction with the Weald Clay are a reddish-brown or chocolate-brown clay. Altbough slightly coarser in grading, it is generally similar in character to the Weald Clay. The Atherfield Clay passes upwards into the Hythe Beds which consist of alternating beds of a hard, grey, glauconitic, sandy-limestone and a grey glauconitic, argillaceous­ calcareous sand or soft sandstone. These beds att asa cap rock to a dominant escarpment within the Wealden District often referred to as the Lower Greensand escarpment. Along much of the escarpment, the Hythe Beds show marked evidence of cambering. Sparse evidence of cambe ring between Hythe and Aldington arises from the post-glacial removal of cambered material prior to abandonment of the cliff line. Copyright Protected http://www.howland.co.uk 142 A.F. Howland ROMNEY MARSH AND THE ABANDONED SEA CLIFFS Romney Marsh and the associated Dungeness Foreland are comparatively recent features which have grown to their present size and shape over the last few thousand years having evolved from a wide shallow bay which ran from Hythe to Romney via Appledore (Lewis 1932, King 1966). Gallois (1965) considered that the marsh originally formed behind a sand bar between Fairlight and Hythe some 3-4000 years BP. This was subsequently breached by a marine transgression which moved the coastline back to its former position along the surrounding cliffs. The bay subsequently again silted up and the earlier marsh deposits formed the buried peat which can now be found throughout the area. Additional evidence (Howland 1974) supports the inference of Horman (1938) that cliffs at Lympne were progressively abandoned by the sea in an easterly direction. THE GEOMORPHOLOGY AND SLOPE STABILITY OF THE WEALD CLAY Mass movement along the Lower Greensand escarpment has been recorded since early time up until recent decades (Skempton and Weeks 1976, Weeks 1970). That movement is still occuring between Hythe and Aldington is evident from the tilting of trees and the building up of soil behind trees and masonry walls: a recent slip (post 1966) has occurred on the slope at Lympne to the west of St Mary's Church (TR 128344). The setting of this section of escarpment is such that many of the variables which might otherwise affect the degradation processes are constant. The height of the slope is consistent at about 95 metres, the regional geological dip is shallow and virtually normal to the strike of the slope. The major geological units within the slope are of fairly constant stratigraphic thickness and are consistent in character along the slope. The slope is continuous with a constant aspect and so is likely to have experienced the same climatic conditions along its length. The base of the slope is bordered by marsh suggesting a similar style of abandonment has occurred throughout. Thus the only important variable which might affect the slope form is the time since abandonment. If the slope form reflects the slope processes, comparison of the form along the escarpment may give a measure of the variation of these processes with time. Five areas were chosen along the slope from Hythe to Aldington and a sixth, at Linton Park to the south of Maidstone, was also included as its inland character provided a comparison with potentially similar slopes in the marsh study area (Fig 1).